Wall Street regulators say they have not found conclusive evidence so far of suspicious trading related to the attacks of the World Trade Centre on September 11th, though some market activity remains baffling.

"There have been tons of allegations, none of which so far have led to a link to anyone likely to know about the terrorist attack," an unnamed regulator told the Wall Street Journal. "We continue to pursue everything we hear about, and are analysing massive amounts of trading data. But it's still too early to come to any meaningful conclusions."

Many reports of short-trading in airline and insurance stocks by terrorist groups before the attack have circulated in the US and Europe. Investigations have centred on evidence of an unusual drop in stock prices which could have been caused by short-sellers seeking to profit from the fall. Short-sellers borrow stock and sell it hoping to buy it back at a lower price and pocket the difference.

No conclusive evidence of terrorist group involvement was found in an examination of monthly short-interest data released on Friday for transactions made from September 10th and settled by September 13th.

However the Wall Street Journal reported a significant jump in shorting activity in some vulnerable shares. XL Capital, of Bermuda, a property-casualty insurer that expects claims of $600-$700 million (€656-€766 million) from the attacks, reported that short interest shares rose to 5.1 million, nearly three times the mid-August figure.

While this has baffled securities experts, as the company had reported strong performance, short interest in American International Group, the biggest property-casualty insurer in the US, fell by half.